Contact boundaries and stress results averaging

[Mechaero2006]

Dear all,

I recently completed successfully a 3d connection problem where two steel plates are connected with a bolt and subjected to tension/bending. The question arises from the observation that the element stress results through the "extrapolation" option need to be "averaged" or remain "linear" depending on the contact boundary option. I explain:

With contact boundaries set to "Discrete" and "job results"-->"Element results" option set to "all points" then during post processing I need to "average" the stresses through the settings-->Extrapolation in order to see the correct results. To avoid this, "Element results" option has to be set to "centroid" from the "job results" menu. Then the correct stresses appear with the extrapolation set to "linear" (default setting, no intervention from the user).

With contact boundaries set to "Analytical" exactly the opposite happens: Element results are set to "all points" and the stresses appear correctly without "averaging". If the "centroid" option is active then the stresses have to be "averaged".

Does anyone know/guess why the contact boundaries affect the element stress results? As far as I understand in both cases the results are correct, however "averaging" is needed to have the results matching as explained above.

I sincerely hope that my question isn't a confusing one; should any additional info is needed please ask me to provide more details.
Thank you in advance for your kind assistance.

 

[Mechaero2006]

Further investigation showed that the problem described above was evident only when the model was subjected to a pure tensile load (single point force). There was no significant difference in stress results between discrete and analytical contact in other loading conditions (such as bending). Maybe it has to do with the contact detection options.